The use of chlorine as a sanitizer or disinfectant for various water supplies and various types of equipment, like food processing equipment and medical equipment, such as a hemodialysis unit, is common. Because the amount of available chlorine in an aqueous solution relates directly to the disinfecting or sanitizing activity of the solution, a test which rapidly and accurately measures available chlorine is important.
The available chlorine family is comprised of compounds which, when in aqueous solution, yield solutions of hypochlorous acid. The available chlorine family is further divided into compounds containing free available chlorine and compounds containing combined available chlorine. The sum of free available chlorine and combined available chlorine is termed total available chlorine.
Free available chlorine encompasses chlorine-containing compounds in aqueous solution such as hypochlorous acid, hypochlorite ion, and, in strong acid solutions, free chlorine. The use of free available chlorine as a disinfectant for water supplies and equipment is widespread because of its low cost, convenience, and effectiveness as an antiseptic agent in relatively low concentrations. For example, free available chlorine is used as a disinfectant in a majority of hemodialysis centers.
Combined available chlorine, also termed bound available chlorine, mainly encompasses organic chloramines, which release only a small amount of free available chlorine in aqueous solution. Chloramines are formed from chlorine reacting with amine compounds in water. The amine compounds can be an impurity in the water or arise from ammonia added to water with chlorine during water disinfection. Ammonia and chlorine are added to the water to form chloramines which stabilize chlorine from decomposition and/or evaporation, and also increases the bacteriocidal potency of chlorine. Depending on the ratio of chlorine-to-ammonia and the acidity of the water, chloramines formed from chlorine and ammonia are a mixture of monochloramine, dichloramine, and trichloramine at various ratios. Although monochloramine is the main chloramine of concern due to its toxicity, removal of all chlorine is essential for safe and effective operation of a dialysis water purification system.
Conventionally, combined available chlorine has not been considered an effective disinfectant or sanitizer. Accordingly, prior chlorine assays have focused on assays for free available chlorine, i.e., the active disinfectant. For example, assays disclosed in Rupe et al. U.S. Pat. No. 4,092,115 and Ramana et al. U.S. Pat. No. 5,491,094, consider combined available chlorine as an interferant in the assay for free available chlorine, and the assays have been designed only to measure free available chlorine. However, in some applications, it is important to assay for total available chlorine.
For example, chlorine is used in hemodialysis centers to sanitize hemodialysis units because chlorine is an effective and economical sanitizing agent. It is important to clean and disinfect a hemodialysis unit between each dialysis session to prevent pathogen contamination from patient to patient. However, chlorine also is a very toxic compound that can cause hemolysis even when only a trace amount of chlorine diffuses from the hemodialysis unit into the blood of an individual. Therefore, if an assay for residual chlorine in a hemodialysis unit detects only free available chlorine, a potentially toxic amount of combined available chlorine, which slowly generates free available chlorine, can be present to adversely affect an individual subsequently connected to the hemodialysis unit. Trace amounts of free available chlorine also can adversely affect filtration membranes of the hemodialysis unit.
Combined available chlorine is considered highly toxic because of its electronic neutrality and ability to penetrate cell membranes. In a municipal source water, combined available chlorine always exists in various proportions relative to total available chlorine. Combined available chlorine is formed in a reaction of free available chlorine either with amine compounds, which are present as contaminants in the source water, or with ammonia, which is added to the water with free chlorine to stabilize the chlorine and to increase the bacteriocidal potency of the chlorine disinfectant.
With respect to a dialysis unit, all chlorine species in a water supply are removed before the water can be used in hemodialysis. Chlorine removal is usually performed by passing the water through a water purification tank containing activated carbon, and then through a reverse osmosis column. The presence of combined available chlorine in the water affects the efficacy of the carbon tank in removing all chlorine species. Knowledge of the concentration of total chlorine is important in designing the water purification system, as well as devising a method of monitoring chlorine in the purified water.
Occasionally, a trace amount of chlorine leaks through the tank. If the chlorine leaking through the tank is all, or substantially, combined chlorine, this suggests exhaustion of carbon tank capacity. However, if the chlorine leaking through the tank contains a high proportion of free chlorine, this indicates the presence of a mechanical defect, such as channelling through the activated carbon inside the tank or an insufficient contact time between the water and the activated carbon. Determination of both the free and combined available chlorine is important in managing the water purification for dialysis.
Therefore, when a sanitizing solution is used in medical or food processing equipment, two critical chlorine levels must be monitored. First, the free available chlorine concentration must be sufficiently high to perform a sanitizing or disinfecting function, i.e., at least about 1000 ppm (parts per million) free available chlorine is needed to effectively sanitize equipment. Typically, a chlorine concentration sufficient for equipment sanitization is about a 1 to 10 volume dilution of a 5.25% (by weight) sodium hypochlorite with water, to provide a solution containing about 0.5% to about 0.6% (by weight) sodium hypochlorite, i.e., about 5000 to about 6000 ppm chlorine. During the sanitizing process, the sanitizing solution is assayed periodically to ensure that sufficient free available chlorine is present to sanitize the equipment.
After the sanitizing function is completed, and before use, the equipment is rinsed with water to flush residual chlorine from the equipment. The rinse water also is assayed for available chlorine to ensure that the level of residual available chlorine is below the maximum allowable level, e.g., 0.5 ppm as recommended by the Association of Advancement of Medical Instrumentation (AAMI). In practice, the residual available chlorine concentration is essentially zero, or at least below the previous lowest detectable levels of about 0.1 to about 0.2 ppm, i.e., equivalent to a 1 to 100,000 water dilution of 5.25% (by weight) sodium hypochlorite.
A chlorine test strip having a broad range chlorine sensitivity of up to 5,000 ppm can be used to monitor the presence and absence of chlorine during the sanitizing and cleaning process. However, when assaying the water quality of a purified source water for use in a dialysis unit, a much more stringent water quality standard is used.
Municipal water normally contains 0.5 to 3.0 ppm chlorine in order to suppress the growth of microorganisms during the transport of water from the water treatment plant to consumers. Because free chlorine is not stable and quickly decomposes during water distribution, ammonia commonly is added with chlorine to generate chloramine. As previously stated, chloramine is less reactive than free chlorine, and is much more stable. Therefore, chloramine is a more effective disinfectant because of its long-lasting reactivity. For the same reason, chloramine also is considered a more toxic water contaminant, particularly when the water is to be used in hemodialysis units.
The Association for the Advancement of Medical Instrumentation (AAMI) standards rate water containing greater than 0.1 ppm of chloramine as unsafe for hemodialysis use. In almost all dialysis facilities, the tap (e.g., the source) water, therefore, is routinely filtered through a bed of activated carbon to remove any trace amount chlorine and/or chloramine. The effectiveness of the carbon tank is constantly monitored to ensure that the chlorine total/chloramine level is less than 0.1 ppm, and preferably zero. Because of the extremely low concentration of chlorine and, particularly, because of the slow reactivity of chloramine, no presently available test strip allows a convenient quantitative assay for low concentrations of total available chlorine.
Commercial assay systems are available for assaying hemodialysis units for available chlorine. One assay utilizes tablets or dry powder, and another utilizes dry chemistry test strips. Each assay has advantages and disadvantages, and neither assay satisfies the different testing requirements needed for a hemodialysis unit.
The tablet method has good sensitivity (e.g., 0.1 ppm) and is less expensive per assay. However, the tablet method is more cumbersome to perform and requires more technician time. The dry chemistry test strips usually are not as sensitive as the tablet method and can cost more per test. Nevertheless, the strip test is very easy and convenient, particularly when operating a mobile hemodialysis unit. In most hemodialysis centers, the test strip is used as a screening test for residual chlorine, whereas the tablet method is used for more critical water testing. Because of the differences in test requirements, most hemodialysis centers are forced to stock both the tablet and dry chemistry test systems.
Dialysis facilities rely on the tablet method using a liquid chemistry assay procedure for the determination of low concentrations of total available chlorine in water. In the assay, chloramine is allowed to react with iodide ion to form iodine, which in turn reacts with N,N-diethyl-p-phenylenediamine (DPD) indicator to generate a light pink color. The sensitivity of the test is achieved by increasing the depth of view through a long verticle path of the reaction tube. As previously stated, the test is cumbersome and time consuming. Conversion of this test method to a strip format is difficult because such a long depth of view is not feasible.
HI SENSE REAGENT STRIP, marketed by Serim Research Corporation, Elkhart, Ind., is another commercially available chloramine test device. In the test, chloramine first is reacted with an iodide ion solution to form iodine, which in turn reacts with a leuco dye impregnated on a membrane. The test requires multiple operation steps and about approximately eight (8) minutes to complete. The test provides only qualitative results by showing positive or negative with no color. The test lacks quantitative information on the chloramine level and also lacks the convenience of a single step test and short reaction time of 60 seconds or less, which are essential for a rapid routine assay.
In the parent application, now U.S. Pat. No. 5,811,254, a single assay for assaying both the high and the low available chlorine concentration range was disclosed. That disclosure showed that it was possible to cross the 10,000 fold difference in chlorine concentration between a working sanitizing solution and a residual chlorine concentration, and detect and differentiate concentration levels over a broad range. Using the test strips makes it difficult to detect ultralow levels of chlorine, i.e., below 0.1 ppm. As opposed to the dip-and-read, it now was found that by repeatedly exposing the strip to the test sample, the strip can detect lower levels of chlorine. Yet, sensitivity still is limited because the process also causes rinsing of the indicator from the strip, thereby making the color on the reagent pad barely visible. Furthermore, the iodide ion in the test pad also is quickly rinsed from the test pad to such a low concentration that catalytic conversion of chloramine by iodide became very slow. This makes the prior test strip less reliable for low-level total chlorine testing.
In order to overcome these problems, it is necessary to immobilize the indicator on the substrate matrix to prevent it from being rinsed off the test pad, and the iodide ions must be anchored in such a way that they are slowly released during the test. The present disclosure sets forth a practical, approach to solving these problems. In particular, the present invention is directed to providing an assay for total available chlorine that is capable of quantitatively measuring total available chlorine concentration over the range of 0 to about 2 ppm, and especially about 0.05 to about 1.50 ppm.
The present invention, therefore, is directed to an assay method and device that can be used to assay for total available chlorine, both free and bound available chlorine, present at 2 ppm or less. Accordingly, a test strip can be used to test for residual chlorine in the rinse water after cleaning the hemodialysis unit or for the chlorine content of the source water for the dialyzer. As illustrated hereafter, the present test strips have a good sensitivity and a detection range over 0 to about 2 ppm total available chlorine with a continuous color response. Such a determination provides important information with respect to whether potentially harmful amounts of available chlorine are present in rinse water or dialysis water.
The present method of assaying for total available chlorine in an aqueous test sample yields trustworthy and reproducible results by utilizing an indicator reagent composition that undergoes a color transition in response to a low concentration of total available chlorine, and not as a result of a competing chemical or physical interaction, such as a preferential interaction with another test sample component. For example, the present indicator reagent composition has sufficient sensitivity to quantitatively detect 0.1 ppm or less total available chlorine.
In accordance with the present invention, an indicator reagent composition can be incorporated into a carrier matrix to provide sufficient sensitivity and color differentiation to assay for total available chlorine concentration over the range of 0 ppm to about 2 ppm, and typically about 0.05 to about 1.50 ppm. In addition, although dry phase test strips have been used to assay for chlorine concentration, no dry phase test strip has been used to quantitatively assay for total available chlorine over such a low concentration range. Examples of prior disclosures relating to assaying for chlorine include Storm U.S. Pat. No. 3,718,605; Reiss U.S. Pat. No. 4,938,926; Ross, Jr. et al. U.S. Pat. No. 4,049,382; Frant U.S. Pat. No. 5,300,442, Harp U.S. Pat. No. 5,362,650; O'Brien et al. U.S. Pat. No. 4,904,605; and J. D. Johnson et al., Analytical Chemistry, 40(13), pages 1744-1750 (1969).